Copper plumbing tubes are a common fixture in any building. They seem to go on forever. The metal has been used for plumbing fittings since the 1930s, remains energy efficient and conforms to stringent modern quality standards. The production process for copper fittings is an interesting story.
1. Raw material
The raw material for copper tubing can be scrap or refined copper. Scrap copper comes often as the recycled copper wire that is stripped of insulation. It may contain some impurities such as zinc and nickel. Cathode copper is a higher-quality material that is produced in vast electrolytic cells. These cells can be the size of football pitches and refine copper from smelting furnaces. The final product is extremely pure. Copper ingots that look like large bricks are produced from remelted scrap or cathode copper. These are used by foundries and brass mills.
2. Melting, casting and piercing
The raw materials are melted at about 1,260 to 1,316 degrees Celsius, well above copper’s melting point. Oxygen is put in contact with the melt to react with impurities that in turn float to the surface. The melt is stirred with large green wooden poles. This is an ancient trick whereby the wood burns, vaporises and speeds up the chemical reactions. The metal is ready to be cast when it reaches specific purity standards. The oxygen content of the copper is controlled by adding phosphorous to the furnace. This reduces the proportion of copper oxide in the melt. The molten metal is transferred from the melting furnace to a holding furnace where it is cast into large logs. It is covered with graphite powder at this point to stop oxidation. There is a continuous casting process that can be done vertically or horizontally. The logs, called billets, are reheated to make them pliable. A piercing mandrel, a type of pointed rod, is driven lengthwise through their centres to create a shape that eventually becomes a plumbing pipe and is used in pipe fittings.
3. Extrusion and drawing
The next step is to heat the billet again and place it in a chamber with an extrusion press. This has a die at one end and a hydraulically driven ram at the other. As the ram moves forward it forces the copper over the top of the mandrel and casts a long hollow tube. The tube’s diameter is reduced by drawing it through a series of steel dies. This process continues until the desired diameter is achieved.
4. Annealing and finishing
Copper tubing can be sold in the hard, drawn state or annealed into a soft condition. Tubing sold as coils is usually annealed in a continuous furnace operating at 704 degrees Celsius. The eventual tubing has a matt finish compared with the shinier hard tubing. The tubing is finished by cleaning it of any drawing lubricants or other contaminants.
Copper tubing has been used in pipe fittings for over eight decades and remains the most popular metal for this use today. The manufacturing process is not particularly high tech but produces a high-quality product.